Police have released a photograph of a suspect, Peter Reeve, 64, and have vowed to "not rest" until he is captured.

Detectives from Essex police launched a round-the-clock search involving armed response units and scores of officers after Dibell was shot in Redbridge Road at 3.40pm on Monday.

The force said it had found the suspected gunman's vehicle at a nearby road junction and warned members of the public not to approach Reeve, who is believed to be armed.

The manhunt, which is being conducted with Kent police, continued overnight, with local officers deployed to protect the public and maintain a strong presence in the area where the killer struck.

A white tent stood on the spot where Dibell was shot and more than 13 police vehicles were parked nearby, including a van full of armed officers.

A local resident told the BBC that she and her next-door neighbour heard a woman screaming then what they thought were five or six gunshots. Another local, Eric Long, 80, said he saw the gunman from his window.

He told reporters: "I saw him standing with some kind of revolver. He was chasing somebody, firing as he went."

"Next there was, it looked like a blonde guy, taking aim and made a couple of shots at him.

"Another guy flew past the house, sort of running, and that I assume now is the off-duty policeman.

"And then there were some more shots and then about five or six minutes after that there was a lady who seemed to be screaming in the middle of the road, and then we had all the police."

He added: "It was like one of those Clint Eastwood-type things where you've got the guy sort of crouching slightly on his knees and he's just taken a shot.

"I thought it was an airgun at first but obviously when I spoke to the police they said to stay in the house because it wasn't, it was live."

The other man injured in the attack was receiving hospital treatment for gunshot wounds to the leg, police said.

Jim Barker-McCardle, the chief constable of Essex police, described Dibell as a "brave, respected and experienced neighbourhood officer" and said it appeared that he was killed after deliberately intervening in an "extremely deadly situation".

Although Barker-McCardle did not reveal the fine details of the dispute, reports suggest the off-duty officer had challenged the gunman who was chasing a man and woman along Redbridge Road.

Speaking about Dibell at the force's Chelmsford headquarters on Monday, the chief constable said: "This officer is a highly regarded and much loved colleague who has been a important part of Essex police for 10 years.

"Policing is a family and we are all hurting. This dreadful event reminds us all that policing is a mission that ultimately some die for.

"We will not rest until we have Peter Reeve in custody; the investigation will continue round the clock."

"The death of our colleague in Essex is deeply sad news for the police service," he said. "PC Ian Dibell died while knowingly walking towards danger, as police officers are called upon to do on behalf of the public they serve.

"The courage he showed in stepping up to do his duty, without thought for his own safety, sums up what it means to be a police officer."

Mark Smith, chairman of the Essex Police Federation, added: "Constable Ian Dibell was a dedicated professional officer who, even though off duty, has paid the ultimate sacrifice for his bravery and selfless actions in trying to protect his local community.

"No words can help those closest to PC Dibell at this terribly distressing time but I hope they will find some comfort in knowing that the loss of this brave man is mourned by the entire police family throughout the United Kingdom."